It's true, I've risked losing all my
friends by doing this project.I suppose it finally proves I am
a geek at heart! (Is there a helpline I should ring?)

Seriously though - having spent many
hours staring at dials and needles in an aircraft tracking many of these
beacons, one builds a little semi-human relationship with them. I think
this is called anthropormophism. Anyway, it seemed kinda
cool to go visit them and build a gallery.

(Actually, my interest in transmitters
seems to go way back to my childhood. I distinctly remember buying the
IBA Yearbook and carrying it with me when we went on UK family holidays.
I used to ask my Dad to divert so I could visit and photograph the TV
masts.)

So.....like every good pilot should,
I got the Long/Lats of the NavAids from the UK AIP at National
Air Traffic Services. I then put them into streetmap.co.uk
to get exact positions for each facility. Having printed a couple of map
pages for each one - I keep the whole lot in the car. Every time I'm near
a NavAid I've not yet been to - I'll photograph it and add it here. First
one was 24th Feb '03. I'll keep going until I've done them all. (I'm mainly
concentrating on the higher power Enroute facilities and not all the airfield
ones.)

Actually, the project was a very
relaxing thing to do while at flying school during the IR phase. I loved
the mini-adventures of going to new places, the driving and the navigation
exercises of trying to find the facilities! They're often in beautiful
countryside and the surroundings can be quite picturesque. I also managed
to see friends around the country at the same time.

There's definitely something
very retro about the beacons both in physical design and of course the
analogue technology employed. I guess they'll all be replaced by Digital
Satellite Navigation in the end and my NavAids Gallery will just become
a museum to the past!

ps; Despite his protestations,
I must give a special thanks to my Oxford Radio Nav lecturer - Stuart
Dawson - who got me through the exam. His legendary Lancashire pronunciation
of anything ending "ation" (think attenuation, modulation, navigation,
integration, configuration......) will never be forgotten by us all.

When I was doing my JAA Radio Navigation
exam I couldn't find much on the Net about VORs. I wanted to know a little
more about their operation. I did some research and here's my summary of how they work.....

The general principal of VOR technology
is to have two seperate 30Hz modulations on the VHF transmissions from
the VOR station. The VOR is arranged such that one of
the 30Hz signals remains in the same phase at all reception positions
around the VOR (Reference signal). The other 30Hz signal received (Variable
signal) will differ in phase by exactly the angular displacement of the
receiver around the VOR from the Zero radial. The aircraft receiver demodulates
the two 30Hz signals and simply compares their phase difference.

An audio ident in morse code at
1020Hz is also transmitted to enable pilot identification of the VOR tuned
and verification of its servicibility. Some VORs have speech audio channels
carrying ATIS or other ident info.

There are two main types of VORs in operation (ie two types of actual ground installation) but the aircraft receiver is the same for both. The receiver is unaware which type of ground station is in use - it experiences the same effects from both. It's the method of creation that differs.

Conventional
VOR - CVOR

The CVOR employs a rotating directional
antenna. Consider for a moment a directional antenna which has a transmission
pattern of one broad peak and one broad null in the horizontal plane.
If we were to feed this antenna with a VHF carrier and also rotate the
antenna at 30 revs/second (1800RPM) - think of how an AM receiver would
view this rotation.An AM
receiver would see a carrier amplitude modulated by a sine wave of 30Hz
- the phase of which would be determined by the receiver's position around
the station.

A practical CVOR doesn't actually
spin the aerial at 1800RPM (although the earliest ones did!) - it uses
electronic switching of an aerial array to achieve the effect. Additionally,
the reference signal is transmitted by FM modulating it onto a 9960Hz
subcarrier (deviation +/-480Hz). The reference signal provides the receiver its comparison
to station North. The receiver compares the AM 30Hz variable phase signal
recovered with the decoded 30Hz reference signal from the FM subcarrier and
determines the radial position from North on which the receiver lies.

Doppler VOR
- DVOR

The DVOR is a later and improved
design of VOR which suffers less from siting errors. The CVOR requires
a clear area of at least 1500ft in radius. The DVOR is more practical
in crowded areas or where there are tall buildings. However, it's a big
structure - around 100ft in diameter!

The DVOR reverses the useage of
the two 30Hz signals. However, by also reversing the direction of it's
rotating variable signal it produces exactly the same result in the receiver.
The receiver has no "knowledge" that it's a DVOR as opposed
to CVOR it's receiving and operates as normal.

In the DVOR the main VHF carrier
is AM modulated at 30Hz - providing the Reference signal. This is transmitted
from a central omnidirectional antenna and has the same phase all around
the VOR for any receiver.

The effect of a 9960 FM modulated
subcarrier is created using the Doppler effect by emplying a switched
array of antenna arranged in a circle of diameter 44ft. (This distance
being the exact amount to provide +/-480 apparent frequency shift in the
subcarrier.)

Imagine a carrier of FcMHz AM
modulated at 30Hz on the central antenna. Then imagine an array of an
even number of aerial elements arranged around the cental aerial in a
circle of diameter 44ft. (Typically 48 are used.) The VOR controller presents
the subcarrier as sidebands on the opposite ends of an imaginary arm.
Pairs of opposite aerials are switched in to form a rotating arm at 30Hz
(1800RPM). The opposite aerials elements carry sidebands of (Fc+9960Hz)
and (Fc-9960Hz).

From the receiver's perspective,
there's a constant phase 30Hz AM modulation on the main FcMHz carrier
but there also appears to be a 9960Hz subcarrier which is in turn frequency
modulated at 30Hz. The sidebands will appear to be frequency modulated
at 30Hz by +/-480 Hz due to the rotation and subsequent 44ft variation
in distance between transmitting aerial and receiver causing Doppler Shift
as the transmitting "arm" rotates. Of course, the phase of the
30Hz frequency modulation on the subcarrier (with respect to the reference
signal) will depend on the receiver's angular position around the VOR.
Hence, the same receiver comparison will result in the receiver's radial
position being established as in the CVOR.

DME - Principle
of Operation

Left - Oxford
Airport stand-alone DME

Right - Colocated DME at LAM DVOR

General
Principle

DME is a secondary radar system which
determines slant distance between aircraft and ground station.

The airborne equipment simply measures
the time that the radio signals take to travel from aircraft to ground
station and back as the means to determine the range .

Detail

DME operates in the UHF band between
962 and 1213MHz. It employs 1Mhz spacing providing 252 channels. The aircraft is equipped with an Interrogator
and the ground station is termed a Transponder.

As DME facilities are usually
co-located with VORs or ILSs facilities and are used in conjuction with
each other - the UHF DME channels are paired with VHF VOR & ILS frequencies.
Operationally, the pilot has only to set the VOR/ILS frequency and the
DME interrogator is tuned automatically to its correct channel pairing.

After selection, the aircraft's interrogator
transmits a stream of pulse pairs and simulataneously starts a range-search.
The ground based transponder receives the pulse train and re-transmits
them after 50uS delay on a frequency which is +/-63MHz from the interrogation
frequency.

The airborne interrogator identifies
it's own stream of pulses and measures the time interval between the start
of its interrogation and the response from the ground transponder. Accuracy
is to +/- 0.2nm. The ground transponder can handle approximately 100 aircraft
interrogators at once. (2700 pulse pairs per second)

The interrogation pulses are 3.5uS
wide and are transmitted in pairs at 12uS (X-Channel) or 36uS (Y-Channel)
intervals.The pulse pairs are transmitted by the interrogator at random
intervals to differentiate themselves from the pulses of other aircraft.
Pulse pairs are used to avoid the vunerability that would occur with a
single pulse system - namely - misidentifying single pulses from lightening,
other radar, ignition systems etc as valid.

As the ground transponder re-transmits
all the pulses received from all aircraft the airborne interrogator sets
up a "gate" to filter the received stream and allow through
only it's own random pulse stream. As the distance between aircraft and
transponder changes the interrogator adjusts it's gate to match. The DME
interrogator is now said to have achieved lock-on and is in tracking mode.

NDB/ADF
- Principle of Operation

Picture: Woodley NDB

General
Principle

Automatic Direction Finding (ADF) simply
provides the relative bearing of a basic ground based Non Directional
Beacon (NDB) to the fore/aft axis of the aircraft by using a directional
antenna assembly in the aircraft.

The NDBs are only basic AM transmitters
which have been specifically sited and are continuously monitored for
servicability. (There's no reason in principle why one couldn't use the
ADF equipment to track to a known AM MW/LW broadcast mast.)

Detail

The NDB transmitter emits a vertically
polarised AM modulated carrier in the LF or MF band. Allocated frequencies
are 190KHz - 1750KHz. The carrier is modulated with an Audio ident in
Morse Code.

Long range NDBs may have useful
ranges of more than 50nm - possibly several hundred miles over oceanic
areas. Low power Locator NDBs are often found on airfields and may only
have a 10 - 25nm range. Accuracy is +/-5deg.

The basic principle of ADF systems is
to use a directional loop aerial and a non-directional sense
aerial to determine beacon position. Imagine a loop aerial placed
in the plane of the transmitted radio wave. As the loop is rotated the
current induced in it will vary. It will be maximum when it's in-line
with the carrier and minimum when the loop is perpendicular to the carrier.
As the loop continues to rotate beyond 180 degrees the current induced
will be in the opposite sense. Hence, there will be two null positions
where the loop is perpendicular to the transmission.

In order to eliminate one of the null
positions (and so provide an unambiguous fix on the position of the beacon)
an additional sense aerial is employed. The simple diapole sense aerial
has a unidirectional (circular) polar diagram. By ensuring that the field
from the sense aerial is in phase with one side of the loop aerial and
then electronically adding the signals together - a resultant cardioid
polar diagram of the whole assembly is established. The Cardioid now
has one null when the loop aerial is rotated and can be used to establish
the beacon's bearing.

A further detail remains. Because
the null in the single cardioid pattern is not precise enough for ICAO
navigational requirements the technique of rapidly reversing the polarity
of the sense aerial is used. By switching the sense aerial at 120Hz a
pair of cardioids is produced and a also more defined null.

Practical System

As an external airborne rotating loop
antenna assembly is impractical, actual installed ADF systems use a fixed
loop aerial assembly and Goniometer. The fixed loop assembly has four
elements - two aligned with the aircraft fore/aft axis and two aligned
perpendicular to fore/aft axis. The electrical fields induced in the elements
are transmitted to a set of coils in the Goinometer - effectively recreating
a local model of the fields experienced by the external loop elements
inside the instrument. A further search coil detects the direction of
the beacon and can drive a pointer.